Old diamonds and the upper crust

Abstract

We report zircon oxygen isotope ratios and reconnaissance Ti-in-zircon concentrations, guided by cathodoluminescence image studies, for detrital zircons up to 4.34 Ga from the Narryer Gneiss Complex of Western Australia. Zircon oxygen isotope results bolster the view that some Hadean (>3.85 Ga) zircon source melts were enriched in heavy oxygen, a sensitive proxy for melt contamination by sediments altered in liquid water. Zircon crystallization temperatures calculated from Ti concentration in pre-3.8 Ga zircons yield values around 680°C in all cases except for one lower value in a 4.0 Ga grain. Elevated zircon δ 18O values reported here and elsewhere, combined with low minimum-melt crystallization temperatures, and analysis of zircon/melt partitioning of rare earth elements (REEs) provide mutually consistent lines of evidence that the Hadean Earth supported an evolved rock cycle which included formation of granitic water-saturated melts, extensive continental crust, hydrosphere-lithosphere interactions, and sediment recycling within the first 150 million years of planet formation.